1. Field of the Invention
This invention relates to the treatment of contaminated waste streams, more particularly to municipal or industrial wastewaters. This invention is especially useful in the biological processing of contaminated wastewaters.
The treatment of contaminated waste streams from municipal or industrial sources involves a sequence of processing steps for maximizing the purification of the waste stream at minimum cost. Generally, biological treatment processes involve sequential processing steps to decontaminate waste streams. One such process involves the biological treatment of dissolved organics and ammonia in water. One of the most common biological treatment processes employed is the activated sludge process.
The activated sludge process is a conventional wastewater treating process which can produce the highest degree of biological treatment in reasonably compact facilities at the present time. Two stages of treatment are generally used in the activated sludge process. In the first stage, a contaminated waste stream is contacted with activated sludge, which comprises microorganisms and, if desired, a powdered adsorbent medium. The most commonly used adsorbent medium is activated carbon. The microorganisms feed on the contaminants in the waste stream and metabolize these contaminants to form cellular structures and intermediate products. After decontamination is completed, the decontaminated stream and activated sludge are transported to a clairifier where the sludge particles and any adsorbent medium present are separated by gravity from the decontaminated stream. Typically, a portion of the sludge and carbon is recycled back to the first stage for contact with the incoming contaminated waste stream. Unfortunately, gravity separation does not facilitate separation of the activated sludge from the surface of the absorbent medium adsorbent. The remainder of the sludge and adsorbent medium can be further processed to remove excess water and to thicken the sludge-adsorbent mixture. The thickened sludge is permitted to digest, that is, the microorganisms feed upon their own cellular structures. The digestion step stabilizes the microorganisms. U.S. Patent No. 4,073,722 teaches dewatering, thickening, and digestion of activated sludge and powdered activated carbon mixtures.
A variety of powdered adsorbent materials are used in biological treatment systems. Powdered activated carbon is the material most commonly used in biological treatment processes. Although it is known that powdered adsorbent materials enhance the performance of activated sludge processes, the exact mechanism is still not clear. Activated carbon enhancement appears to include the mechanisms of improved buffering, increased biological surface area, decreased sensitivity to toxic substances, improved phase separation, and adsorption. Only the last mechanism requires regeneration of activated carbon for reuse. Adsorption is most important when the system is operated at low solids retention times (SRT). As the SRT of operation increases, the role of adsorption decreases and the other mechanisms dominate. Even at low SRT, when adsorption is important, the recovery the reuse of nonregenerated carbon improves operation. The art has not recognized that it is feasible to achieve enhanced performance by recycling wasted or spent carbon that has been recovered by means other than by conventional regeneration. Practicing a once through operation without reclaiming and recycling carbon is costly and presents significant disposal problems.
Conventionally, thermal regeneration has been the solution to avoiding disposal of spent activated carbon. One thermal method proposed for regenerating powdered activated carbon involves the wet oxidation of mixture of biological solids (biomass) and spent powdered carbon. Another is controlled flame incineration. Wet oxidation at elevated temperatures and pressures destroys the volatile portion of the biological solids and oxidizes the organic substances adsorbed on the surface of the powdered carbon to restore the carbon's adsorptive capacity. The resultant regenerated powdered carbon is recycled as part of an aqueous slurry to the treatment process.
There are, however, drawbacks to the wet oxidation regeneration process. These include, among others, the high potential for the destruction of the powdered carbon, the high cost of operation and the dangers associated with the operation of high pressure reactors. The wet oxidation reactor must be carefully controlled to avoid excessive temperatures, high oxygen levels, or extended residence times, which all result in loss of the powdered activated carbon. High carbon losses significantly increase the overall cost of the biological treatment system of which it is a part.
The present invention overcomes the problems of prior art regeneration processes and provides an efficient and economic biological treatment process that employs a combination of two innovative unit operations. The first of these is the concept of reusing or recycling a spent powdered adsorbent medium that has not undergone regeneration. The second operation is a unique recovery process whereby the spent powdered adsorbent medium is recovered from a mixture of activated sludge and spent adsorbent medium through a nondestructive disassociation procedure that utilizes the process of cell lysis.
2. Information Disclosure Statement
Process for biological treatment of waste streams are well known in the art, especially those that employ activated sludge. Also the use of powdered adsorptive materials to enhance the effects of activated sludge is well known. Powdered activated carbon is the most commonly used of the adsorptive materials available and can be used in conjunction with aerobic, anaerobic, or anoxic microorganism containing processes.
The treatment of municipal or industrial wastewaters is disclosed in U.S. Pat. No. 4,292,176 (Grutsch et al.) wherein powdered activated carbon is used to enhance a multistage biological treatment process. In addition to the wet oxidation process for regeneration of powdered activated carbon, the art recognizes other regeneration processes such as biological oxidation, anaerobic digestion, and furnance treatment. For example, U.S. Pat. No. 4,407,717 (Geletzke et al.) teaches a process for treating wastewaters with a powdered adsorbent in a biological packed tower or trickling filters. This reference also teaches that a portion of spent adsorbent containing solids may be regenerated through either thermal or biological treatment processes. Regeneration through biological treatment requires up to 30 to 60 days to complete a regeneration cycle. The Geletzke reference does not teach the recovery of powdered adsorbent using cell lysis.
An example of such a biological regeneration process is described in U.S. Pat. No. 3,803,029 (Blecharczyk), which provides a process for generating or reactivating a particulate adsorbent contained in a fixed bed by biologically degrading adsorbed impurities. Regeneration is accomplished by directing aerated activated sludge upwardly into the fixed bed of spent adsorbent. Alternatively, treated liquid waste is separated from a mixture of activated sludge and activated carbon and then the adsorbed contaminants on the carbon are degraded by a regeneration procedure involving aeration of the sludge. In each case the regeneration procedure is carried out until complete digestion of entrained materials is accomplished and the adsorptive potential is fully restored.
Regeneration of powdered adsorbent support mediums is also disclosed in U.S. Pat. No. 4,626,354 (Hoffman et al.) wherein a process for treating high strength biochemical oxygen demand (BOD) containing liquors is taught. In this reference it is taught that spent adsorbent material may be regenerated using controlled atmosphere furnance combustion, wet oxidation, or other processes which will destroy the biomass and regenerate the adsorptive property of the support medium. Hoffman et al. further teaches that less-than-complete regeneration is undesirable because the adsorbent will now be able to adsorb color from the water phase.
Another treatment process is disclosed in U.S. Pat. No. 4,328,105 (Arbuckle) where suspended solids and adsorbable contaminants are removed from an influent liquid. This process is characterized by the use of porous polyurethane particles having particulate adsorbent material within the pores of the polyurethane. Adsorbate loaded adsorbent is regenerated using mechanical agitation and a "clean" regeneration liquid. No biomass is present during the regeneration.
Although the art has understood the need to use adsorptive particulate materials in biological treatment processes, it has failed to provide an activated sludge process that is enhanced by the addition of adsorptive materials and that includes both the reuse of unregenerated adsorbent materials and a novel recovery procedure that utilizes cell lysis and not destructive thermal treatment.